The present invention relates to a driving unit using an electrostrictive element, and more particularly to a driving unit which drives mechanical members such as a diaphragm and a shutter or a shutter blade having the function of a diaphragm blade by deformation of the electrostrictive element produced when a voltage is applied to the element, and still more particularly to a novel driving unit using an electrostrictive element which is adapted to obtain a desired deformation quantity without influence of the magnitude of an initial deformation quantity due to hysteresis of the element.
It is well known that when a voltage is applied to, for example, a piezoelectric ceramic having a fixed end through electrodes laminated thereon a free end of the ceramic is deformed in accordance with the voltage applied thereto. Recently, a driving unit employing a characteristic of such an electrostrictive element is known in which deformation of the element produced when a voltage is applied thereto is used as a mechanical driving source to drive, for example, a shutter and a diaphragm or a shutter blade having the function of a diaphragm blade.
As a patent of a prior art pertinent to the present invention, the inventor can indicates as existence of, for example, U.S. Pat. No. 4,609,275.
The U.S. Pat. No. 4,609,275 discloses an exposure control unit for a programming shutter which employs an electrostrictive element having a fixed end and a free end which is deformed when a voltage is applied to the element and the deformation of the element is used to open shutter blades having the function of the diaphragm. A voltage increasing with the lapse of time in interlocked relationship with the opening operation of the shutter blades is applied to the electrostrictive element so that the element is gradually deformed and the shutter blades are also gradually opened. The electrostrictive element is then short-circuited at a time when a proper exposure is obtained so that the exposure operation is immediately terminated.
The electrostrictive element operates in the same manner as a capacitor in a circuit. Once electric charges are stored in the electrostrictive element, the element maintains its deformation condition by the stored charges even after removal of an applied voltage. The element is restored to its initial state by discharge of the stored charges. When the electrostrictive element is employed as a driving source for mechanical members, and more particularly when the element is used as a driving source of a mechanism requiring exact positioning accuracy, the element exhibits an undesirable characteristic due to existence of hysteresis characteristic thereof. That is, even if the voltage across electrodes of the element is the same, the deformation quantity of the element in the case of the stored condition of charges thereto is different from that in the case of the discharged condition.
Accordingly, when the electrostrictive element is used as the driving source for the mechanical members and more particularly when the element is continuously operated, the initial deformation of the element is different depending on operation intervals of the element and the deformation quantity obtained is not constant even if a desired voltage is applied to the element. When the element is applied to, for example, a driving mechanism of the shutter blade, an opened position of the shutter blade is not constant even if a desired voltage is applied to the element and hence correct exposure control can not be attained. Such a problem that the deformation quantity obtained finally is not constant due to the magnitude of the initial deformation quantity even if a driving voltage is the same occurs in common in the case where the electrostrictive element is used as the driving source to position the mechanical members as well as in the case where the element is used as the driving source of the shutter blade.